Such marker lights of the flush-mounted type are previously known, e.g. from SE-C-7803611-8 DK-C-113 912 and EP-B1-22106. The flush-mounted type has of course the advantage that the marker light does not have any protruding portions, and it can therefore be located at will on or adjacent to a runway or a taxi strip and can be run over by the wheels of air craft or other vehicles.
However, a problem in this connection is the cleaning and the snow-clearance of the runways, which requires the use of steel scrapers on the cleaning vehicles in order to obtain the desired result, especially when ice is present, which is often the case due to the heat emitted by the light device. Such steel scrapers often cause scratches and other damage to the upper, light emitting surface of the cover plate. In order to prevent such damages, it is proposed in the above mentioned SE-C-7803611-8 that hard particles be embedded into the light emitting, active zone of the cover plate, these particles protruding upwardly somewhat above the transparent material. Hereby, the light emitting, active zone of the cover plate is of course protected from damage in connection with snow-clearance by means of steel scrapers, but the hard particles inevitably cause an undesirable reduction of the light intensity and uncontrollable diffuse spreading of the emitted light.